In the manufacture of a printed circuit board (PCR), in a first (multi-step) stage a "bare board" is prepared and in the second (multi-step) stage, various components are mounted on the board. The bare board is often a multi-layer board, generally comprising a first, inner-layer which is usually an epoxy-bonded fibreglass sub-layer clad on one or more usually both sides, with a conducting sub-layer. The conducting sub-layer usually comprises conducting material which is metal foil and most usually copper. The conducting material is generally a discontinuous sub-layer in the form of the circuit pattern. On at least one, and generally on both sides of the inner layer, an outer (or further) layer is attached. An outer layer will comprise at least a layer of an insulating material, again, generally epoxy-bonded fibreglass which is bonded to the conducting sub-layer of the innerlayer. The outer layer may additionally comprise at least one conducting sub-layer. The outer layer or layers are insulating layers or are adhered to the conducting sub-layer of the first inner layer with an insulating layer or sub-layer innermost to the first layer.
A multi-layer board therefore comprises a laminate in which conducting sub-layers are separated by insulating layers or sub-layers. The laminates are formed by adhering layers together. The layers which form the laminate may be conducting or insulating only or more usually are made up from sub-layers: at least one insulating and at least one conducting sub-layer. Generally conducting sub-layers are only provided on an insulating sub-layer due to their fine, fragile nature.
Various difficulties have been noted in trying to permanently bond the copper circuit pattern surface and an insulating organic substrate together. The copper surface tends to oxidise on exposure to the atmosphere to form a tarnished layer on its surface. If the next layer is adhered directly to this tarnished layer of copper, the bond will be weak and eventually will fail.
The most commonly used method for overcoming this problem is to remove the tarnish layer and form a strongly adherent copper oxide layer.